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| Fig. 1: Energy Schematic for a Thermionic Converter. The electrons escape from the hot cathode (left), and accumulate on the cold anode (right). The bending of the energy bands of the cathode allows some electrons to tunnel out. (Source: Simon Swifter) |
Thermionic energy conversion is a process by which thermal energy can be transformed into usable electricity. While such transformations have been historically limited by low efficiencies, recent advances have made thermionic energy conversion a viable source of energy generation in certain applications.
A thermionic converter at it's simplest form consists of two pieces of metal at different temperatures, held at different electrostatic potentials. The colder piece of metal, called the anode, is where the electrons are collected, and the hotter piece of metal, called the cathode, are where the electrons are generated. The cathode is heated to a very high temperature, and this allows electrons to gain enough thermal energy to escape from the cathode by overcoming the energy barrier, or work function, of the metal (Fig. 1). The ejected electrons find their way to the anode, and because the anode is held at a higher potential, a portion of the energy that the electrons gained from being heated can now be used in an external electrical circuit. A simple analogy is water being pumped uphill, where the voltage corresponds to the height of the hill, and the energy is gained from the water rushing downhill. The process of heating the metal corresponds to pumping the water to the top of the hill, where a hotter cathode corresponds to a stronger water pump. Likewise, the higher the hill, the more energy can be extracted by letting the water run downhill, and the higher the voltage the more energy can be extracted from the electron in the external circuit.
Thermionic energy conversion is not a novel concept. In 1958, Wilson described a simple thermionic converter with a energy conversion efficiency of 9.2%, which means for every Joule of heat injected into the cathode, .092 Joules of energy can be extracted from the anode in the form of electricity. [1] This converter was made with a cathode and anode of tungsten and molybdenum respectively, enclosed in a glass tube pumped with cesium gas to facilitate the thermionic emission process. [1] Since then many techniques and processes have been improved and innovated to improve the feasibility of his technology.
There are certain advantages to thermionic converters over other energy converters, such as a traditional Carnot heat engine, which converts heat into mechanical energy in the form of work. One benefit of the thermionic process is that there are no moving pars in the system, which allows for very long operational lifetimes. Furthermore, thermionic converters can be fabricated at a much smaller scale than the Carnot engine, which opens the door for possibilities of thermal energy conversion at the microscale.
The primary challenge of thermionic energy conversion is low efficiencies when compared to Carnot engines. Large mechanical heat engines, such as steam turbines, have a practical efficiency of about 40%. [1] Thermionic converters, however, have a practical efficiency limit of around 20%. [1] A major contributor to the low efficiency problem is the work function of the electrodes. Work function is a product of the material surface and serve as a barrier to electron emission, and thus it is desirable to have low work functions to avoid large energy barriers. Furthermore, the available energy to be extracted is proportional to the difference in work functions of the two electrodes. Having two low work function electrodes while maintaining a large difference between the electrodes is an ongoing challenge.
Another drawback to efficiency in these systems is what is known as the space charge effect. Electrons are negatively charged, and thus repel each other. As thermionic emission occurs, electrons disperse in the area between cathode and anode, and repel other electrons that are ejected from the cathode, preventing them from reaching the anode. This drastically decreases the efficiency of the converter. [2] Early converters mitigated this issue by introducing ionized (positively charged) gases to the area between cathode and anode, however the converters still suffered from low efficiencies.
Since Wilson's simple thermionic converter in the 1950s, several advances in thermionic conversion technology have made it a viable commercial option in various applications. One way to improve performance is to optimize the amount of space between the cathode and anode. Lee et al. described how reducing the distance between cathode and anode could mitigate the space charge effect without the need for ionized gas or plasma. [2] However, if the gap is too small heat can leak directly from the cathode to anode, and losing potential energy. By balancing these two effects, Lee et all found that optimal gap widths ranged from 900 nm to 3 microns, depending on th temperature of the hot cathode. [2] The calculated maximum conversion efficiency is 23% for a cathode at 1500°K, making it viable for energy conversion in a variety of applications. [2]
Another advance is the use of nanofabrication of the cathode and anode surfaces to increase the efficiency of the conversion process. By patterning materials at the nanoscale, engineers can adjust the work function of the anode and cathode. Low work function materials are required to extract as much energy as possible from the emitted electrons. One example is Lanthanum Hexaboride, which can significantly lower the work function when compared to typical electrode materials. [3] Furthermore, fabrication of the converter in thin layers or fabrication of nanowires on the emitter surface allow electrons to tunnel from the cathode to the anode without overcoming the work function, adding to the overall efficiency of the device. [4,5]
The benefits of thermionic energy application make it desirable for a number of applications. One such application is increasing the efficiency of power plants that burn fossil fuels. These plants operate at temperatures over 2000°K, but the turbines generally need only 800-1300°K. [5] The extra heat could then be used to power a thermionic converter, increasing the overall efficiency of the plant. In 1973, Rasor Associated estimated that the efficiency of a nuclear fusion plant could be increased from 41.3% to 47% by implementing thermionic converters. [5] Another application of thermionic energy is optimizing energy expenditure in a house. Excess heat from water boilers or natural gas burners could be used as electricity, simultaneously conserving energy while saving homeowners money on utilities. Several companies on the west coast are currently attempting to commercialize thermionic energy conversion for both utility and residential applications.
Thermionic energy conversion is a promising technology that allows for the extraction of usable energy from heat sources. Challenges including space charge and the difficulty of finding low work function materials have suppressed the usefulness of this technology in the past, but recent advances have made it a viable method of energy conversion in industrial and residential environments. Several companies are attempting to commercialize the technology to compete with traditional forms of heat energy conversion such as steam turbines.
© Simon Swifter. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] V. C. Wilson, "Conversion of Heat to Electricity by Thermionic Emission," J. Appl. Phys. 30, 475 (1959).
[2] J. H. Lee et al., "Optimal Emitter-Collector Gap for Thermionic Energy Converters," Appl. Phys. Lett. 100, 173904 (2012).
[3] Ahmed, H. & Broers, A. N. Lanthanum hexaboride electron emitter. J. Appl. Phys. 43, 21852192 (1972).
[4] T. Zeng, Thermionic-Tunneling Multilayer Nanostructures For Power Generation, Appl. Phys. Lett. 88, 25 (2006).
[5] K. A. A. Khalid, T. J. Leong, and K. Mohamed, "Review on Thermionic Energy Converters," IEEE Trans. Electron Dev. 63, 2231 (2016).